Sunday, December 12, 2010

Questions From Odd Velocity Patterns on the DMX Radar

I noticed a lot of pretty patterns last night looking at the radar data as the snow moved through the midwest. Some of the best came from the Des Moines radar (KDMX). They were running in VCP 31 for much of last night--a good VCP for observing the structure of snow, but a bad one for getting good velocity data. Here's the reflectivity field at 0.5 degrees

Excellent structure with multiple snow bands can be seen. However, a look at the velocity image for this time shows a pretty complicated picture.

Fig 2 -- 0.5 degree base velocity from KDMX at 405Z, Dec. 12, 2010.

In an earlier post, I talked about how VCP 31 had issues measuring velocity because it used a longer radar pulse. In fact, the maximum unambiguous velocity it can detect is around 11.5 meters per second, or around 22 knots. Looking at the above image, you can see I've also overlayed surface observations. Surface winds are already at 20-30 knots across much of Iowa--so we're already approaching the threshold. Wind speeds typically increase with height as well. So, as the radar beam goes out (and up in height), it's going through faster winds--faster than its maximum unambiguously detectable velocity. That's why we're seeing so many layers of opposing color here--the radar is "aliasing" velocities higher than 22 knots to the opposite direction. Then the radar aliases again once the velocities get past 44 knots--only now it's aliasing back to the correct direction, but at the wrong speed. Each multiple of that maximum velocity (the Nyquist velocity) triggers another aliasing of the velocities. And based on wind profiles, we get well above 70 knots of wind aloft--that's at least three aliases of velocity going on in that image above.

Let's go to a higher tilt (3.5 degrees) and zoom into the velocity image a bit.

Here you can see there is a lot going on in the lowest 10,000 feet of the atmosphere. We know based on the surface observations that the winds near the surface are out of the north-northwest. This means we'd generally expect green colors (meaning incoming air) to the north and red values (meaning outgoing air) to the south. Of course, the winds are so close to the Nyquist velocity that they're almost immediately aliased--in fact most of the large red area to the north of the radar should actually be green. And that green blob directly north of the radar? That's where the velocities have been aliased again--they're back to going the right direction, but the magnitude is off by a factor of two times the Nyquist velocity since they've been aliased twice. Similar things are happening to the south of the radar, where everything should probably be red.

Anyhow, one thing we can do based on the image is draw the wind directions based on the location of the zero isodop. The zero isodop is the line of "zero" velocities (the gray line) that separates the inbound and outbound air (it can actually be any place where zero velocities are showing up but the zero isodop is usually the one I just described). The zero isodop follows the line where the wind velocity is perpendicular to the radar beam. Since the winds are roughly north to south in the image above, when the radar beam scans to the east or west, there will be a point when the winds are blowing perpendicular to the radar beam. Since the radar can only measure air moving toward or away from the radar, it measures no velocity when the air is moving perpendicular to the radar beam. Thus we get a line of zero velocity in every radar image that represents all the places where the wind is blowing perpendicular to the radar beam. We can see that in the above image, the "true" zero isodop is the gray line of zero velocities that squiggles all the way across the image and goes through the radar.

Since along this line we know the direction the radar beam is pointing, we can draw arrows perpendicular to the radar beam along the zero isodop and extract wind directions. Since the radar beam increases its height off the ground the further it goes out, we can extract a vertical profile of winds from this zero-isodop analysis. I've drawn a few sample wind direction arrows in the image below (I make no claims about the wind velocity, just the direction).

We can see that because the zero isodop "squiggles", the wind directions also change with height. In fact, it looks like there are three different layers here--immediately near the radar, the winds are out of the north-northwest. Then there seems to be this small layer (which works out to between 2500-5500 feet, about) where winds are more out of the north-northeast. Then above that, there's another layer where winds are more northerly. Or course, between these layers there is directional wind shear going on. I've labeled these wind shear places (where the wind is changing direction) in the image below:

Now, from our thermal wind arguments, we know that veering winds (winds turning clockwise with height) tend to imply warm air advection and backing winds (winds turning counterclockwise with height) tend to imply cold air advection. Well, that's kind of funny. We know for a fact at the surface that lots of cold air advection is going on. Temperatures are in the teens and single digits in northwestern Iowa and in the mid 30s in central Iowa. With north-northwesterly winds near the surface, this would mean that cold air is moving into central Iowa and we're seeing cold air advection. However, these wind shifts would imply that there is some sort of weak warm air advection going on just above the surface.

What's causing this? To be honest, I'm really not sure. Some sort of latent heat release from melting precip? The surface temperatures in central Iowa are very close to freezing, or even slightly above it. Perhaps some of the snow is melting right before it hits the surface and this is causing some kind of warming. It's an intriguing possibility. We don't see much of a melting layer in the reflectivity image:

Perhaps there is some slight enhancement of the reflectivity close to the radar (i.e. at low levels) which would indicate melting precipitation. But it's difficult to tell. Another thought is that this could be the so called "warm conveyor belt" of air wrapping around the upper-level trough bringing slightly warmer air (most likely from the northeast) over the shallow surface layer. This seems slightly more plausible--after all this is the wrap-around region of the cyclone--but at the same time I would have expected the warm conveyor belt to be a bit higher off the ground than 2500 feet. But, perhaps this is the case.

The radar can also somewhat verify itself when it comes to areas of wind shear. There's another product called the spectrum width product. Think of this as a standard deviation of velocity measurements at a given point. Since the radar processor takes an average of several different measurements to determine the velocity at a particular point, the spectrum width is more or less just the standard deviation of all those measurements. Therefore areas of high spectrum width tend to be areas of higher uncertainty with regards to what the velocity is. This could mean that the wind velocity is changing rapidly in that bin, like in areas of high shear or turbulence. Here's the spectrum width image at the same time as the above images.

This is actually the image the originally inspired this post, as I thought it a remarkably amazing image. It's not often on radar that you see a nicely spiraling pattern like that (well, outside of a hurricane). There are distinct bands where the spectrum width is peaking. So, I thought, where do these spectrum width maxima cross the zero isodop? I added black lines to the annotated velocity image to show where this happens.

Oddly enough, those maximum bands of spectrum width seem to correspond almost exactly to the locations where we're seeing that directional wind shear (and inferred temperature advection) in the velocity image. But this makes sense--if the wind directions are changing rapidly with height, the radar's estimate of the wind velocity may not be as good. So the spectrum width image seems to confirm these areas of wind shear.

What I can't exactly describe is why the spectrum width bands seem to be spiraling. If there was one uiniform layer of veering winds and then another uniform layer of backing winds, I would have expected concentric circles, one at each level where the velocities were changing, and not a spiral. This probably says that there's some sort of tilt to these layers in the horizontal, but I can't say much more beyond that. If anyone else has any ideas, by all means, please let me know...

This sort of isodop analysis to determine winds is actually used by an algorithm in the radar product generator to create a product called VAD winds. It uses model soundings (and some math on the velocity field) to calculate a vertical profile of winds (both speed and direction) from the radar (though it assumes that the winds are horizontally uniform across the radar domain--not always the best assumption). The VAD profile can be looked at in "meteogram" format like shown below.

Fig 9 -- VAD winds from KDMX at the times listed on Dec. 12, 2010.

The profile corresponding to the volume scan of all the images above is the profile that is furthest to the right. And what do you know--winds start out out of the northwest at the surface, then veer to becoming northeasterly from about 2000-5000 feet, then back to more northerly above that. So the VAD winds also confirm these slight directional wind shifts--and verify my analysis of the velocity image.

So my analysis of the image seems to be consistent. But my interpretation of the images is rather loose. What really is causing these subtle variations in the wind direction (since it's not just showing flat out cold air advection)? Is there some sort of warming due to precipitation melting near the surface? Or is this wind shift simply due to friction with the land causing odd wind direction shifts near the surface? Or is that the warm conveyor belt showing up as subtle shifts in the velocity profile? I have my random thoughts, as explained above. I'd welcome any thoughts anyone else has. I just thought that there was a really pretty spiral image in the spectrum width and this is where it led me...

Also, just for comparison, the sounding from Omaha from 00Z (four hours before and over 100 miles to the west--well into the cold air) looked like this:

They also show a change from north-northwesterly to northerly winds somewhat above the surface. Does this confirm the warm conveyor belt hypothesis? The temperature profile does show warmer temperatures above the cold layer near the surface. Lots of puzzles with this one.

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Completed graduate school at the University of Washington, now a postdoctoral researcher at NCAR in Boulder. The thoughts and opinions expressed on this blog are solely those of the author and are do not necessarily represent the positions of NCAR.
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